This invention relates generally to the field of sterilization and more particularly to sterilization based on the use of ethylene oxide.
Sterilization by the application of boiling water or steam to the article to be sterilized has been carried out for many years. More recently there has arisen in certain fields, such as in medicine and in space exploration, the need to employ a different sterilant because certain articles used in these fields cannot withstand the temperatures or the moisture associated with steam sterilization.
One sterilant that has become widely used is ethylene oxide because not only is it an effective sterilant but also its residues volatize relatively quickly from the article sterilized. Although ethylene oxide may be used by itself to carry out the sterilization, this is generally not done because ethylene oxide is highly flammable. Instead, ethylene oxide sterilant is generally used in a mixture with a flame retardant. The flame retardant, however, must complement the properties of the ethylene oxide or the beneficial effects of the ethylene oxide will be lost. Over the last two decades the flame retardant of choice for use with ethylene oxide in a sterilant mixture has been dichlorodifluoromethane, known in the industry as CFC-12. The most commonly used sterilant mixture comprises 27.3 mole percent (12 weight percent) ethylene oxide and 72.7 mole percent (88 weight percent) CFC-12. This mixture is commonly referred to in the industry as 12-88.
Recently a problem has arisen in the use of CFC-12 because it is one of the chlorofluorocarbons believed to cause significant damage to the ozone layer in the upper atmosphere. Accordingly, worldwide reduction and elimination of the use of CFC-12 is now underway. This has created a problem for the use of ethylene oxide as a sterilant.
Alternative flame retardants have been proposed, but they are not as effective as CFC-12 in that less ethylene oxide can be mixed with these alternatives before the mixture becomes flammable. For example, the maximum amount of ethylene oxide that may be mixed with either 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) or 1-chloro-1,2,2,2-tetrafluoroethane (HCFC-124) before the mixture becomes flammable is 23 mole percent. For pentafluoroethane (HFC-125), the maximum amount of ethylene oxide that can be added to HFC-125 before the mixture becomes flammable is 19 mole percent, and for 1,2,2,2-tetrafluoroethane (HFC-134a), the maximum amount of ethylene oxide is 12 mole percent.
Further, HCFC-123 and HCFC-124 both contain chlorine and therefore may adversely affect the Earth's atmosphere.
As mentioned above, ethylene oxide may be used by itself as a sterilant. However the explosion danger of such use makes it acceptable for only a relatively few applications at locations which have experienced and sophisticated handlers available at all times.
One flame retardant which is known for use with ethylene oxide is carbon dioxide. However, because of the characteristics of carbon dioxide, a non-flammable ethylene oxide-carbon dioxide mixture contains less than 40 percent of the ethylene oxide per unit volume as does 12-88. Thus, sterilization must be carried out either at higher pressures or for longer contact times. Furthermore, the large difference in the vapor pressures of ethylene oxide and carbon dioxide causes the mixture to separate upon withdrawal from the storage tank or cylinder, raising the danger of delivering a sterilant mixture rich in carbon dioxide, which will not sterilize, or rich in ethylene oxide, which is explosive.
It has now been found possible to provide an improved sterilant mixture employing ethylene oxide which overcomes the deficiencies of the known sterilants and to provide an improved sterilization method using a sterilant mixture employing ethylene oxide which overcomes the deficiencies of the known sterilization methods.